God, that's a good question. The NTP daemon in OSX is based on the one in FreeBSD, so you can find the settings for it in the /etc/ntp.conf file. The default for this is shown below (actual server will be different depending on which one you've selected):-
server time.euro.apple.com minpoll 12 maxpoll 17
As you would expect, the minpoll and maxpoll variables indicate the minimum and maximum intervals that the ntpd daemon checks the NTP server to synchronise the clocks. However, the variables are set as 2 to the power of whatever number is specified.
So the minimum polling interval in the default shown above is 2*12 or 4096 seconds (1 hr 11 min 36 seconds), and the maximum is 2*17 or 131072 seconds (roughly 1.5 days). The actual interval will be as close to the minimum value as your network connection will allow.
As for your other question, the actual clock offset between your computer's clock and the NTP server is shown in the /var/run/ntp.drift file. Providing MacRumors' servers are also synchronised to an NTP server, your clock should remain synchronised with MR.
Hope that answers everything 🙂